Digital holographic microscopy is one of the most widely techniques for quantitative phase imaging in recent years. It can realize the object reconstruction of numerical focus, and has the ability of quantitative phase analysis at sub-wavelength level. However, it is also affected by the optical diffraction limit, so that the wavelength of light source and the limited aperture size will limit the spatial resolution, and can't break through the lateral resolution limit of half-wavelength between two points. Therefore, synthetic aperture digital holographic microscopy is proposed for improving spatial resolution of the optical system, the method includes: placing diffraction grating to modulate high-frequency information of object, mechanical movement of image sensor to capture the wide field diffraction information, and using angular multiplexing to capture the high frequency information of object at various angles in order to enhance the spatial resolution.
At present, the lateral resolution can reach 90 nm (nanometer) in the technical field of synthetic aperture, which is utilizing complex amplitude deconvolution approach to complete the sub-micron wavefront reconstructing of object. It is a method of synthesizing aperture to use the mechanical galvanometer to rotate incident angle of the beam to scan the object. However, such mechanical scanning will cause additional perturbations and can't precisely adjust the incident angle, which will limit the axial phase accuracy.
Therefore, the structured illumination digital holographic microscopy is proposed to realize the synthetic aperture method without mechanical scanning, which method includes using a diffraction grating for the incident light separating into the zero order (0th) and a positive one and negative one order diffraction (±1st) terms, placing a pinhole filter to filter out the negative first-order diffraction (−1st) term, filtering out the zero order diffraction (0th) to output low frequency plane reference wave, and interfering with positive one order diffraction (+1st) term of the object diffracted wave to form a hologram. In addition, there is using a liquid crystal spatial light modulator to replace the diffraction grating, which reconstructs wavefront information by shooting a series of holographic images and using a phase retrieval algorithm.
However, the structured illumination digital holographic microscopy system requires using complex optical architecture to avoid crosstalk of individual frequency bands, and using projection patterns of the structured illumination to implement temporal phase shift, which can't perform synthesis spectrum expanding for resolution enhancement of the object in single exposure. It will make the measurement system sensitive to environmental perturbation so that the axial phase accuracy is reduced, and the complex recording process does not achieve spatial resolution enhancement.
In addition, due to the spectrum overlap of bandpass spectrum of object wave, it needs through taking at least two sheets of phase shift patterns to separate the spectral overlap, and thereby solving the problem of spectrum overlap and reconstruct the bandpass spectrum of different high frequency and low frequency. However, such time multiplexing will make the axial phase accuracy susceptible to environmental vibration and therefore affect the signal-to-noise ratio of the reconstructed images.
However, at present, there are some drawbacks in digital holograph technology for industrial applications. It is necessary to develop a novel structured illumination digital holograph technology to solve the above problems.